As is known, a twin-engine or three-engine helicopter has a propulsion system comprising two or three turboshaft engines, each turboshaft engine comprising a gas generator and a free turbine which is rotated by the gas generator and is rigidly connected to an output shaft. The output shaft of each free turbine is suitable for inducing the movement of a power transmission gearbox (referred to in the following by the abbreviation PTG), which itself drives the rotor of the helicopter which is equipped with blades having a variable pitch.
Furthermore, it is known that, when the helicopter is in a cruise flight situation (i.e. when it is progressing in normal conditions, in AEO (all engines operative) mode, during all the flight phases apart from transitional phases of take-off, landing or hovering flight), the turboshaft engines operate at low power levels, below their maximum continuous output (hereinafter MCO). In some arrangements, the power provided by the turboshaft engines during a cruise flight can be less than 50% of the maximum take-off output (hereinafter MTO). These low power levels result in a specific consumption (hereinafter SC), which is defined as the relationship between the hourly fuel consumption by the combustion chamber of the turboshaft engine and the thrust provided by the turboshaft engine, which is approximately 30% greater than the SC of the MTO, and thus a reduction in the efficiency of the gas turbines (or an increase in the SC).
In order to reduce this consumption during cruise flight (or during holding on the ground for example), it is possible to stop one of the turboshaft engines and to put it into a mode known as standby. The active engine or engines then operate at higher power levels in order to provide all the necessary power, and therefore at more favorable SC levels.
In the following, “economical flight phase” will denote a flight phase during which at least one turboshaft engine is in standby mode, and “conventional flight phase” will denote a flight phase during which none of the turboshaft engines are in standby mode.
In FR1151717 and FR1359766, the applicants proposed methods for optimising the specific consumption of the turboshaft engines of a helicopter by the possibility of putting at least one turboshaft engine into a stable flight mode, referred to as continuous flight mode, and at least one turboshaft engine into a particular standby mode that it can leave in emergencies or in a normal manner, according to need. A transition out of the standby mode is referred to as ‘normal’ when a change in the flight situation requires the turboshaft engine in standby to be activated, for example when the helicopter is going to transition from a cruise flight situation to a landing phase. A normal transition out of standby mode of this kind occurs over a period of between 10 seconds and 1 minute. A transition out of the standby mode is referred to as an ‘emergency’ when a there is a power failure or a power deficit in the active engine, or when the flight conditions suddenly become difficult. An emergency transition out of standby mode of this kind occurs over a period of less than 10 seconds.
A turboshaft engine leaves a standby mode and transitions from an economical flight phase to a conventional flight phase for example by means of a pack for restarting the turboshaft engine, which pack is associated with a device for storing energy, such as an electrochemical store like a lithium ion battery, an electrostatic store like a supercapacitor, or an electromechanical store like a flywheel, which allows both the turboshaft engine to be provided with the energy required for restarting and a nominal operation mode to be reached.
Such a pack for restarting the turboshaft engine in standby has the disadvantage of substantially increasing the total weight of the turboshaft engine. The benefit in terms of fuel consumption obtained by placing the turboshaft engine in standby is thus partly lost by the excess weight brought about by the restart device, the device for storing energy required for the restart, in particular when each turboshaft engine is equipped with an emergency restart device of this type.